What a learner can do afterwards
- Explains why manganate(VII) acts as its own indicator and describes the end point
- Builds the full ionic equation from the two half equations before calculating anything
- Calculates the concentration or the purity of a sample from the titre and the mole ratio
- Explains why the iodine and thiosulfate route needs starch added only near the end
1 · Read
Manganate(VII) is its own indicator in acid solution, so you add no separate indicator at all. The end point is the first faint pink that stays, which is the first drop of extra manganate tinting the flask.
Before any numbers, build the full ionic equation from the two half equations. Manganate takes 5 electrons while each iron(II) gives 1, so 1 manganate reacts with 5 iron(II).
Picture a dissolved iron tablet needing 25.0 cm3 of 0.020 M manganate. That is 5.0 x 10-4 mol of manganate, times 5 gives 2.5 x 10-3 mol of iron, times 55.8 gives 0.1395 g of iron in the tablet.
With iodine and thiosulfate, hold the starch back until near the end. Added early, starch locks onto the iodine and will not let go cleanly, so the finish drags and the end point blurs.
Manganate shows its own pink end, half equations fix the 1 to 5 ratio, titres give mass, and starch waits until near the end.
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